Various forms of altimeters are known in the art. Some of the known altimeter structures utilize microwave signal generating systems and Doppler effect reflections thereof In one known system, disclosed in commonly assigned U.S. Pat. No. 3,860,925 to Ernest F. Darboven, Jr., there is provided a combined system including both a velocity sensor and an altitude measurement system. The altimeter portion of the combined system utilizes echoes of a frequency modulated microwave signal generated on board an aircraft for the velocity sensor, the echoes displaying a Doppler frequency shift as well as a phase shift due to travel to and from the surface of the earth. The disclosure of the above patent is incorporated herein by reference.
In the '925 disclosure there is provided an arrangement for generating a microwave signal, for transmitting the same to be reflected by the earth, and for processing the received echo. In one portion of the processing portion, there is also provided an altitude measuring system which processes the amplitude information of the returned signal to determine therefrom the altitude of the aircraft with respect to ground.
More particularly, in the Darboven patent a double side band (DSB) signal is demodulated to obtain a baseband signal and the resultant amplitudes of the two channels are measured. A block diagram summarizing the Darboven process is shown in FIG. 1. As shown therein, a double sideband signal is received at an input terminal 10 from the Doppler receiver. The signal is passed through sine and cosine channels 12 and 14. The two channels include mixers 16 and 18, respectively, wherein the received (demodulated) signals are mixed with sine and cosine signals 20 and 22, respectively. A pair of leakage elimination filters (LEF) 24, 26 is provided in the two channels, and low pass filters 28, 30 provide output signals for processing by a buffer (not shown).
In the buffer, the two sinusoids provided by the two channels, which have an amplitude difference therebetween, are processed to provide a gate signal having a gate width equal to the arctangent of the amplitude difference between the two sinusoids, thus providing a measure of the altitude of the aircraft. This operation is equivalent to division in a divider circuit 32, providing a tangent of the desired phase angle, followed by conversion by an arctan converter 34 illustrated in FIG. 1. The output of arctan converter 34 is shown as providing an output signal which is the desired altitude gate signal. The description of the two signals after the low-pass filters 28 and 30 is given by the following equations: EQU e.sub.1 =sin (.theta.) cos (w.sub.D t) (1) EQU and EQU e.sub.2 =cos (.theta.)cos (w.sub.D t) (2)
In the above equations, the w.sub.D term is the frequency shift due to the Doppler effect, which is dependent on vehicle speed The phase term, .theta., is due to the time delay of the signal travelling to the ground and back. It is thus directly dependent on altitude. As shown in these equations, the altitude information exists in the relative amplitudes of the two channels. If the outputs of the two channels are divided by one another and the arctangent of the quotient is taken, the phase delay of the echo can then be determined and the altitude obtained.
This method, however, suffers from problems of accuracy. The amplitude terms have to be measured with extreme precision to obtain good altitude accuracy. Analysis has shown that this cannot be done with sufficient precision to meet the accuracy requirements of current applications in a cost effective manner.
Inasmuch as various phase shifts are introduced into the above described system by microwave and RF circuitry used therein, a calibration mode is needed to compensate for such phase shifts Two prior art methods have been used to calibrate the altimeter.
The first involves opening all the microwave switches connecting the receiver/transmitter to an antenna, so that the microwave energy is reflected back into the receiver instead of being transmitted to the ground. Since the signal thus reflected to the receiver possesses no Doppler shift, a RF modulator is required to create an artificial shift therein. This is done because the altimeter, operating in conjunction with a Doppler velocity measurement, requires a Doppler shift to operate. The RF modulated signal can now be fed into the altimeter.
The resultant phase shift measured by the altimeter is thus only that which is caused by the microwave circuitry. To obtain a corrected altitude measurement, the phase shift measured by this process may be subtracted from the total phase shift measured in the Doppler ground return signal obtained during operation of the altimeter, since the Gunn oscillator sees the same signal during the calibrate mode as during the normal operating more.
However, although this method is an accurate approach to calibration of a Doppler altimeter, the prior art implementation of this approach is expensive and bulky. That is, in the prior art it is required to provide an RF modulator and other microwave circuitry to implement such calibration, which increases both expense and equipment requirement for implementation.
A second calibration method involves measuring the phase of a microwave leakage term to obtain a calibration factor. This method is also described in the Darboven patent. The leakage term results from the receiver picking up energy reflected from the microwave switches before it has been beamed to the ground. The result, once again, is a signal without a Doppler shift, with a phase shift which should be dependent only on microwave circuitry. This phase is measured and is then used as a calibration correction factor.
More particularly, for such calibration the signal from the receiver is passed through a bandpass filter which selects the upper, J.sub.1, sideband. The signal is demodulated in a sine-cosine manner in a pair of demodulators to eliminate the J.sub.1 leakage component. Switched capacitor filters, composed of switching matrices and low pass filters, reject the zero speed leakage term which is used for calibration purposes
Although this method is very simple and requires little extra circuitry, testing of this technique has found a number of accuracy problems therein
Another prior art atimeter, utilizing phase shifting of returned Doppler signals, is discosed in prior U.S. Pat. No. 3,111,667. This method is generally referred to as "doubling," and is illustrated by the block diagram shown in FIG. 2. The double-sideband (DSB) signal that is received at terminal 36 from the antenna is first demodulated to baseband at demodulator 38, so that the leakage term will now be at DC. This term is removed by a leakage elimination filter 40 before further processing is done. The resultant signal is remodulated by a remodulator 42 to again form a DSB signal, only now without the leakage. This double-sideband signal is described by EQU e.sub.3 =cos[(w.sub.m +w.sub.D)t+.theta.]+cos[(w.sub.m -w.sub.D)t+.theta.](3)
The w.sub.m term is the frequency which is used to frequency modulate the microwave Gunn oscillator (commonly referred to as the FM frequency). Upon passing this signal through a frequency doubling device 44, there is provided a signal given by EQU e.sub.4 =1+cos[2(w.sub.m +w.sub.D)t+2.theta.]+cos[2(w.sub.m -w.sub.D)t+2.theta.]+cos[2w.sub.m t+2.theta.]+cos[2w.sub.D t](4)
The signal described in equation (4) is sent through a narrow band filter 46, centered at 2w.sub.m, resulting in a reconstructed carrier having a phase which is proportional to altitude. The phase of the reconstructed carrier is measured by comparing the signal to the same signal w.sub.m that was used to frequency modulate the Gunn oscillator. This is done in a loop by using a phase detector 48, together with a phase shifter 49 in a loop as shown in FIG. 2.
In the above described frequency doubling approach, the output of the phase detect or 48 is amplified to derive a motor 50, the output shaft of the rotates to drive the phase shifter 49. The displacement of the output shaft of the motor constitutes the altimeter output quantity, representing the altitude multiplied by a constant.
A significant disadvantage of the above described frequency doubling method is in the accuracy thereof, due to the fact that much circuitry is involved and that any phase imbalance in this circuitry degrades accuracy. Moreover, in view of the prior art use of a loop only in the output phase detection and indication portion of the circuit, while performing all other processing in an open-loop fashion, accuracy of procesing of the signals is further compromised. Another problem with the prior art is the requirement of a demodulator-remodulator circuit before the doubler to eliminate the leakage term. This requires a considerable amount of additional circuitry.
Thus, the prior art has been deficient in providing an accurate, inexpensive, altimeter utilizing reflected Doppler signals generated for velocity measurements. Moreover, where calibration has been proposed, significant additional circuitry has been required. There is thus a need in the prior art for an accurate, calibrated, altimeter which does not utilize extensive amounts of additional circuitry.